Electrical equipment such as gas insulated switchgears is generally configured to house a high-voltage portion within a grounded metal enclosure so that it can ensure safety against access from the outside. In such electrical equipment, the high-voltage portion and the ground portion are insulated from each other with a solid insulator or an insulating gas.
The insulation performance may, however, deteriorate, which can lead to a breakdown, in cases where metal particle is included in the metal enclosure for some reason, the solid insulator deteriorates due to a high-voltage stress, a protrusion is formed that may cause electric field concentration in the high-voltage portion, and a floating electrode that develops a floating potential is formed. Thus, in order to obviate a breakdown in the electrical equipment to improve reliability, it is important to detect a partial discharge that is a sign of deterioration of the insulation performance.
Japanese Patent Laying-Open No. 10-26650 (PTD 1), for example, discloses a partial discharge diagnosis method that includes determining, where two peaks arise in a frequency distribution of time intervals of partial discharge pulses, that conductive foreign matter is producing a firefly, with a high risk of breakdown. The peaks in the frequency distribution occur in a range of 0 to 40 μs and a range of 40 μs or longer. The term “firefly” refers to motion of the conductive foreign matter in the form of a vertical protrusion on the high-voltage portion.
Further, when detecting a partial discharge, it is necessary to distinguish between a partial discharge within the electrical equipment and electromagnetic noise caused by an electrical discharge occurring outside the electrical equipment. The electromagnetic noise caused by the discharge may be generated, for example, at the shield portion of a bushing end through which current is introduced from an overhead wire into the electrical equipment, or over the insulator surface of the overhead wire. Since this electromagnetic noise is similar in frequency spectrum to the discharge within the electrical equipment, it is not readily distinguishable from the discharge by using conventional techniques.
Japanese Patent Laying-Open No. 2005-156452 (PTD 2), for example, discloses a method for distinguishing between a partial discharge within electrical equipment and external noise, by creating a correlation diagram between the voltage phase angle of a particular partial discharge and the time interval to the subsequent occurrence of a partial discharge, and distinguishing between them based on the pattern of the correlation diagram.